As semiconductor devices have been highly integrated and progressively densified, a lithographic patterning on the level of 45 nm becomes possible now. Such patterning can be realized by improved technologies such as an immersion exposure method and a double exposure method, which use ArF instead of a conventional excimer light. However, to cope with a next-generation patterning on the level of 32 nm or even thinner, the exposure technology depending on the excimer light falls short, and an EUV exposure technology, which uses EUV light of which the main wavelength is 13.5 nm, far shorter than that of the excimer light, is considered as a promising solution.
Although this EUV exposure technology has been considerably improved toward its practical use, there remain many technical problems to be solved with respect to a light source, a resist, a pellicle, etc. For example, with regard to the pellicle used to prevent an adhesion of a foreign matter to a photomask, which is a phenomenon that lowers production yield, there are various unsolved problems, and thus the pellicle poses a big obstacle against the realization of EUV exposure technology.
An especially difficult problem lies in that there has not been a clear roadmap toward realizing a development of a material to make the transparent film of the pellicle that does not age with oxidation or the like and is thus chemically stable, in addition to having a high transmittance of EUV light.
Conventionally, a material of a pellicle film for EUV has had various problems. In particular, there are various problems in that organic materials hardly transmit an EUV light and also they are decomposed or deteriorated by the EUV light. Although there exists no material that has a perfect transparency in the wavelength range of EUV light, there are disclosed silicon thin films as relatively transparent films for EUV light (U.S. Pat. No. 6,623,893 and Shroff et al. “EUV Pellicle Development for Mask Defect Control,” Emerging Lithographic Technologies X, Proc of SPIE VOL. 6151 p.1-10 (2006)).
It is desired that these silicon films for EUV application are as thin as possible from the viewpoint of reducing the attenuation of EUV light. However, these silicon thin films, which are made up of a silicon membrane of 20 nm thickness and a rubidium layer of 15 nm thickness, have thicknesses on nanometer order. Therefore, they are very fragile with regard to strength and it is impossible that they are used on their own as a pellicle film for EUV.
For this reason, in order to solve this difficulty inherent to using such silicon thin film as a pellicle film for EUV, there has been proposed a use of a structure which has a honeycomb-like shape and has openings adapted to pass EUV light and is unified with the extremely thin silicon film to reinforce it.
For example, there is proposed a pellicle for EUV which utilizes an SOI (Silicon On Insulator), and the said pellicle has a mesh structure as a honeycomb-like structure for reinforcement of the pellicle membrane for EUV (Japanese Unexamined Patent Publication 2010-256434).
As a mesh structure for reinforcement of the pellicle film for EUV, one can choose from various shapes besides the above-mentioned honeycomb-like structure so long as the intended purpose is met, such as a lattice structure, a plate-like body having openings in an arbitrary shape such as a circle and a polygon. The strength of the structure is determined by a mesh pitch, a width of the thin grid frame defining each mesh opening, and a height (thickness) of the frame, and the narrower the pitch is, the greater the width of the grid frame is and the greater the height (thickness) of the grid frame is, the greater the strength of the structure is.
Since the EUV light cannot pass this mesh structure other than the opening, the opening ratio of the mesh structure ought to be increased in order that the attenuation of the incoming EUV light is minimized. However, as is described above, if the strength of the pellicle film for EUV is desired to increase, the opening ratio of the mesh structure has to be compromised.
In general, the EUV light emitted from a light source in a stepper focuses after following an optical system of the stepper and describes a desired pattern upon a wafer. However, if the light attenuation caused at the pellicle for EUV in the optical system is large, it would be necessary to incorporate a supplemental technology so as to increase, for example, the light emission strength of the light source or the mirror reflectance, or to increase the sensitivity of the resist put on the wafer. As such, the light attenuation caused by the pellicle for EUV would bring unfavorable consequences to all the constituent elements of the EUV optical system, therefore, it has to be avoided as much as possible.
Now, the chief causes for the attenuation of the EUV light coming through the pellicle for EUV are an absorption of the light by the EUV pellicle film and a blocking of the light by the grid frames of the mesh structure, and the latter is determined by the opening ratio. The opening ratio of the mesh structure is determined by factors such as the width of the grid frame, the mesh pitch and the grid frame height (thickness) etc. forming the mesh structure.
Now, the reason why the mesh height is relevant to the opening ratio of the mesh structure is that, in the EUV stepper, the angle of incidence with which the light passes the pellicle film is 4-6 degrees and thus the greater the frame height is, the greater the amount of the light will be blocked. Hence, depending on the mesh height, the resulting shade is changed.
Accordingly, after due consideration of the above-mentioned respective factors, it has been desired to design a structure wherein, while making every effort to avoid the reduction of the amount of the light that reaches the mask, at the same time, the necessary physical strength of the EUV pellicle can be maintained.
The present inventors studied intensively to find a way to simultaneously achieve (1) withholding the lowering of the physical strength of the pellicle for EUV and (2) suppressing the lowering of the amount of the exposure light at the time of patterning through heightening of the opening ratio of the mesh structure installed for reinforcement of the pellicle film for EUV, which are considered as mutually conflicting; and the inventors came to realize that it was possible to improve the light transmittance of the pellicle for EUV without lowering the physical strength of the pellicle for EUV, by shaping the mesh structure in such a manner that, in a shape of longitudinal cross section (a shape of cross section in the height direction) of the grid, the width of the grid frame lessens as the distance from the silicon thin film is increased (See, FIGS. 6 and 7), that is to say it is a decreasing in width (hereinafter, described as “taper-like” in this specification), thereby achieving the present invention.